My first notice of this was while watching football. For a few years now, a technology known as chroma keying has been used to paint the first down lines onto the field to better illustrate this otherwise invisible line for the viewer at home. This had many benefits, including making the game more accessible for people being newly introduced to the sport.

A computer system connected to all of the cameras in the stadium with reference points on the field automatically paints this unobtrusive yellow line onto the green field while keeping it from obscuring any players or objects on the field. The end effect makes it appear that this line is actually on the field at any time, though it's position will move constantly throughout the game.

More recently, I've seen it used commonly during the 2006 Olympic Winter Games during the speed skating competitions. The athlete's home country flag appears to be painted or projected in a static position on the ice in front of the starting line. A similar affect was used at the 2004 Summer Olympics to make it appear that a swimmer's country's flag was painted on the bottom of the pool in the same lane they were swimming in. Once again, the information is conveyed to the audience at home in an unobtrusive way.

A chroma key is the removal of a color (or small color range) from one image to reveal another image "behind" it. The removed color becomes transparent. This technique is also referred to as "color keying", "colour-separation overlay" ("CSO"), "greenscreen" and "bluescreen". It is typically used for weather forecasts. The presenter appears to be standing in front of a large map, but in the studio it is actually a large blue or green background.

It's also interesting to me that this is the primary technology used for the Lightsabers in the Star Wars Films, though this also involved an older technology called rotoscoping.